Ah yes, It's a sure sign of spring when we have this discussion. If you do a search, you'll find a few threads on this subject on cc.com.
I ski unroped on glaciers all the time. I'm sure I would rope up if the conditions were total sketch; however just try skiing roped up once and you'll learn how hard and unfun it is.
It really really helps to have a lot of glacier travel experience. If you don't you should rope up a lot more often.